In a variety of fields such as dental materials, recording materials, and construction materials, curable compositions including polymerizable monomers and inorganic or organic fillers have been conventionally used. Particularly in the field of dental filling restorative materials, since curable compositions can impart a color tone equivalent to that of natural teeth color and is easily operable, curable compositions have been rapidly popularized as materials for restoring teeth that have been damaged by dental caries, fracture, and the like. In recent years, from the viewpoint of enhancing the mechanical strength or enhancing the adhesive force to teeth, curable compositions are also used for the restoration of anterior teeth as well as for molar teeth to which high occlusal pressure is exerted. For example, Patent Document 1 discloses a composite composition for photopolymerization having excellent depth of cure, the composition including a polymerizable vinyl monomer; an inorganic oxide having a refractive index that is lower than the refractive index of a polymer of the vinyl monomer and including particles having a particle size of 0.1 μm to 1.0 μm; and a catalyst capable of initiating photopolymerization by visible light. Furthermore, Patent Document 2 discloses a composite composition for photopolymerization having excellent depth of cure, from which a cured material having an adequate degree of translucency is obtained, the composition including a polymerizable vinyl monomer; a filler having a refractive index that is higher than the refractive index of a polymer of the vinyl monomer; a filler having a refractive index that is lower than the refractive index of the relevant polymer; and a catalyst capable of initiating photopolymerization by visible light.
For the color tone adjustment of conventional curable compositions such as the compositions of Patent Documents 1 and 2, pigment substances, dye substances, and the like have been used, and various color tones have been prepared by varying the mixing ratios of pigment substances, dye substances, and the like, which have different color tones. However, coloration by means of pigment substances and dye substances tends to undergo decoloration or discoloration caused by aged deterioration. In regard to dental filling restorative materials, a curable composition including pigment substances, dye substances, and the like exhibit high color tone adaptability immediately after restoration; however, after restoration, the curable composition is discolored as time elapses, and a phenomenon that the external appearance of the restored site becomes incompatible with the appearance of natural teeth, occurs in many cases.
In this regard, as a structure that can be colored without using a pigment substance, a dye substance, or the like, it has been known that structural color that is expressed by, for example, interference, diffraction, refraction, or scattering or light may be utilized. Generally, color (expression of color) of a material is such that when light having a certain wavelength is absorbed, light of other wavelengths is reflected or transmitted, and when the reflected light has a wavelength in the visible light range, this light is perceived as color. This coloring (hue) is generally color developed by a natural or artificial dye or pigment, and is coloring resulting from energy exchange between light and an object. On the other hand, there is color that is not based on a dye or a pigment but is expressed only by means of the physical nature of light, without exchange of light energy. This is structural color. A synonym for this is “interference color”; however, the interference color is a kind of structural color. The structural color is expressed by diffraction, refraction, interference, scattering, or the like of light. For example, the structural color is expressed in thin film interference caused by coating of spectacles or the like, multilayer film interference caused by a multilayer configuration of thin films, diffraction grating, and a photonic crystal. These have regular structures having constant intervals; however, the structural color is also expressed by scattering caused by, for example, fine particles dispersed in a matrix, which is not a regular structure (all, regarding the structural color, see Non-Patent Documents 1 to 4). For example, Patent Document 3 discloses a recorded matter expressing colored light by means of light interference, the recorded matter having sites where solid fine particles aggregate and arrange on a liquid-repellent surface of a material to be recorded and form a regular periodic structure, in which the standard color chart value of the liquid-repellent surface is 6 or less, and the recorded matter has black color or a dark color having a chroma of 8 or less. Patent Document 4 discloses a color sheet that does not use a coloring dye or pigment and visually presents a chromatic color as a structural color, in which organic or inorganic spherical particles having black color or an achromatic color and having an average particle size (d) in the range of 100 nm to 500 nm as represented on a volume basis, are regularly aligned on a color-developing base material sheet and form a particulate laminate thereon. Coloring by a structural color that utilizes such interference, diffraction, refraction, scattering or the like of light has an advantage that a phenomenon of decoloration or discoloration appearing in the case of using a pigment substance, a dye substance or the like is not observed.
In recent years, in the field of dental filling restorative materials, there is an increasing demand not only for the recovery of occlusion but also for esthetic restoration of the appearance looking like natural teeth. There is a demand for a restorative material which can reproduce not only simple equivalent color tones but also the transparency or color tone at various restoration sites of teeth, and which undergoes less aged deterioration. From this point of view, in both of the compositions of Patent Documents 1 and 2, the particle size distribution of the filler or the relation between the refractive indices of the polymer as a matrix and the filler were not optimized, coloring by a structural color was not always obtained, and since the compositions were colored using a pigment or the like, a phenomenon of decoloration or discoloration caused by aged deterioration occurred.
Thus, Patent Document 5 discloses a curable dental material having high esthetic properties and exhibiting adjustable translucency and high opalescence, the curable dental material containing a monomer having a refractive index of lower than 1.45, an opalescent filler having a refractive index of lower than 1.45, another conventional filler or filler mixture, and at least one selected from the group consisting of a polymerization initiator, a stabilizer, and a colorant, in which the difference between the refractive index of the monomer and the refractive index of the opalescent filler is less than or equal to 0.04, and the average particle size of the opalescent filler is 230 nm±50 nm. However, in the material of Patent Document 5, since the relation between the refractive index of a polymer of the monomer and the refractive index of the opalescent filler is not optimized, coloring by a structural color is not necessarily sufficient, and the range of the average particle size is limited to a range that exhibits opalescence. Thus, it was difficult to reproduce the transparency or color tone at various restoration sites of teeth.
Patent Document 6 discloses a dental composite restorative material which exhibits an opal effect (the same unique light scattering phenomenon as that of mineral opal) and provides excellent esthetic properties, the dental composite restorative material including (A) a polymerizable monomer; (B) spherical silica-based particles having an average particle size in the range of 0.1 μm to 0.5 μm and a standard deviation of the particle size distribution of 1.30 or less; (C) an organic-inorganic composite filler obtained by dispersing these silica-based particles in an organic resin matrix; and (D) a polymerization initiator, in which the difference between the refractive indices of the spherical silica-based particles and a polymer of the polymerizable monomer is 0.1 or less, and the difference between the refractive indices of the organic inorganic composite filler and a polymer of the polymerizable monomer is 0.1 or less. However, in regard to Patent Document 6, the occasion of obtaining an opal effect is substantially limited to a case in which the refractive index of the polymer is larger than the refractive indices of the spherical silica-based particles and the organic-inorganic composite filler, and the opal effect exhibits a bluish color. A bluish-colored dental composite restorative material is suitable for the restoration of an incisal part of a tooth; however, this material is not necessarily suitable particularly for the restoration of the tooth cervix, where reproduction of the hue of the yellowish to reddish dentinal color is necessary. As such, in regard to the material of Patent Document 6, since the relation between the refractive indices of the polymer and the filler is not optimized, it is difficult to reproduce the color tones of various restoration sites of teeth.
As explained above, it is required for a composite restorative material that the color tones of teeth at various treated sites are exactly reproduced. The crown part of a natural tooth is formed from dentine and enamel, and the color tone (hue, chroma, and value) varies from site to site. For example, since an incisal part has a thin dentinal layer and is almost covered with enamel, the incisal part is highly transparent and exhibits a bluish hue. In contrast, the tooth cervix is opaque because the deep part has a thick dentinal layer, and compared to an incisal part, the tooth cervix has high value (lightness or darkness of a color) and high chroma (vividness of color) and has the yellowish to reddish hue of the dentinal color. That is, the chroma and value decrease in the direction from the tooth cervix having a thick dentinal layer at the deep part, toward the incisal part having a thin dentinal layer. Furthermore, the incisal part, which is formed from almost enamel only, shows a bluish hue; however, the other parts show a yellowish to reddish hue in reflection of the hue of the dentinal layer at the deep part. As such, since a tooth has different color tones at different sites, in order to obtain superior esthetic properties for tooth restoration, it is important to prepare a plurality of curable pastes for restoration having different color tones, and to select and use, from among these curable pastes for restoration, a curable paste having a color tone that is most suitable for the actual restored tooth and adjacent teeth thereof (hereinafter, also referred to as “periphery of the restored tooth”) (Non-Patent Document 5).
Such selection of color tone is achieved by a dentist, who uses a shade guide (color sample) that includes a collection of various cured article samples of prepared curable pastes, compares the respective color tones of the samples with the color tone of the periphery of the restored tooth checked by looking into the oral cavity, and selecting a color tone that is felt to be closest to the color tone of the restored tooth.
Furthermore, as long as it is not the case that the damage of the restored tooth is small with a shallow cavity, it is difficult to realize the adaptation of the color tone by means of filling of a single kind of curable paste. That is, if the cavity is deep (for example, Class 4 cavity), the color tone of a tooth is visually perceived in a state in which not only the color tone of the tooth flank part (enamel portion) but also the color tone of the deep part (dentinal portion) that shows through are combined to give a rich gradation. Therefore, a deep cavity is filled by laminating the curable pastes to be filled, by varying the color tone at a certain interval of depth, and thereby this subtle color tone is reproduced. Usually, this reproduction of color tone is carried out such that a plurality of curable pastes for dentinal restoration, which reproduce the color tones of the dentinal portion, are used and laminated from the deepest part (usually, lamination is continued while each layer is cured), and a curable paste for enamel restoration is laminated at the last surface layer (for example, see Non-Patent Documents 5 and 6).
As such, since there are individual differences and site differences in the color tone of teeth, arranging curable pastes having their color tones strictly controlled in consideration of these differences, is substantially impossible because a huge number of curable pastes are needed. Particularly, in the restoration of a cavity in which the dentine is positioned at the surface of deep parts, since the color has high value and high chroma and has a yellowish to reddish hue, and there is a large variation depending on individual differences and site differences, strict control of the color tone as described above is even more difficult.
Patent Document 1: Japanese Unexamined Patent Application, Publication No. S62-86003
Patent Document 2: Japanese Unexamined Patent Application, Publication No. S63-218703
Patent Document 3: Japanese Unexamined Patent Application, Publication No. 2001-239661
Patent Document 4: Japanese Unexamined Patent Application, Publication No. 2004-276492
Patent Document 5: Japanese Unexamined Patent Application (Translation of PCT Application), Publication No. 2007-532518
Patent Document 6: PCT International Publication No. WO 2011/158742
Non-Patent Document 1: SHINODA, Hiroyuki and FUJIEDA, Ichiro, “Shikisai Kogaku Nyumon”, Morikita Publishing Co., Ltd., 1st print published on May 1, 2007, pp. 73-78
Non-Patent Document 2: SAITO, Katsuhiro, et al., “Hikari to Shikisai no Kagaku”, Kodansha, Ltd., 1st print published on Oct. 20, 2010, pp. 118-139
Non-Patent Document 3: The Color Science Association of Japan, ed., “Handbook of Color Science (3rd Edition)”, University of Tokyo Press, published in April, 2011, pp. 1130-1181
Non-Patent Document 4: JIS 28102, 28110
Non-Patent Document 5: MATSUMURA, Hideo and TAGAMI, Junji, rev., “Adhesion Yearbook 2006”, 1st Edition, Quintessence Publishing Co., Ltd., published in August, 2006, pp. 129-137
Non-Patent Document 6: MIYAZAKI, Masashi, “Science & Technique of Composite Resin Restoration”, 1st Edition, Quintessence Publishing Co., Ltd., published in January, 2010, pp. 48-49